Sonic positioning device

ABSTRACT

A three-dimensional ultrasonic position control device suitable for controlling computer displays or robot movements is disclosed. 
     More specifically, signals from an ultrasonic transmitter are received by multiple receivers. The received signals are processed by a processing circuit which provides signals to a computer system for use in controlling a computer display or a robot. This system is particularly adapted to use inexpensive components to provide for a low cost mass producible control device.

BACKGROUND OF THE INVENTION

1. Field

The present invention relates generally to an position control device.More specifically, the present invention relates to a three-dimensionalultrasonic position control device suitable for controlling computerdisplays or robot movements.

2. Art Background

Graphical input devices for computers such as the mouse taught in USP4,464,652 issued to William F. Lapson et al Aug. 7, 1984 are well knownin the field of display systems, and more particularly in the field ofcontrolling cursor positions on a computer display. However, mousesystems require a flat surface on which to operate the mouse and areinherently limited to two dimensions.

Ultrasonics have been used both in continuous wave (cw) applications andin pulse mode (pm) applications to detect positions. While cw techniquesare able to measure distances quite accurately, they have problems withreflections due to multipath phenomena.

Both one-way and two-way pulse mode ultrasonic position detectingdevices are known. Such systems have been used extensively in underwaterapplications such as for tracking torpedoes. U.S. Patent 3,205,475issued to Rene N. Foss describes such a one-way system. A transmitter ispositioned on a torpedo and its position relative to orthoganallypositioned hydrophones is determined. This system is based onsynchronizing clocks on the torpedo transmitter with clocks on thereceiver so that transit times of the ultrasonic signal from the torpedoto the hydrophones can be determined without cabling. However, thissystem relies on accurate synchronized clocks, pre-launchsynchronization and computation of the desired coordinate positionsdirectly from the propagation times of the pulses from the torpedo tothe hydrophones.

U.S. Patent 3,421,138 issued to Pierre Moulin et al describes anotherunderwater positioning system for locating a floating installation or asubmarine. For example, a ship's location is determined by transmittingan acoustical signal from the ship, receiving the acoustical signal atfixed locations on the ocean floor, and radioing back to the ship fromfloating buoys in response to the detections. In the preferredembodiment, this system makes certain approximations which takeadvantage of the fact that in this application the "z" distance, thedepth of the water, remains relatively constant.

U.S. Patent 3,821,469 issued to Albert L. Whesone et al describes agraphical data device which provides digitized three dimensionalpositioning information by responding to the leading edge of the airpropogated shock wave generated by a spark. However this techniquerequires two or three distributed strip-microphone receivers and isgenerally limited to determining positions of a stylus within a volumebounded by the distributed receivers.

Another graphics tablet is described in U.S. Patent 4,124,838 issued toKiss. In the disclosed system a graphics tablet has an acousticallyreflective movable element, two orthoganally positioned distributedstrip-microphones along the axes, and a source of periodic sound wavesat the origin.

A. E. Benner and P. de Bruyne describe a sonic pen system in an articletitled A Sonic Pen: A Digital Stylus System, which appeared in the IEEETransactions on Computers, June 1970. This system is used as a two orthree dimensional computer graphical input device. However this systemalso uses a spark transmitter and distributed strip-microphones.

An article titled "The Lincoln Wand" by Roberts et al in the 1966 FallJoint Computer Conference AFIPS Proceedings describes a threedimensional ultrasonic input system utilizing four transmitters fixed ina square and a hand-held pen containing a single receiver. The multipletransmitters are activated in sequence, allowing for sound echoes to dieout between activations, which results in an inherently long cycle time,and a relatively slow response time.

A device for measuring three dimensional coordinates of models is taughtin U.S. Patent 3,924,450 issued to Uchiyama et al. In this system acomputer is used to calculate the position of an end of a pointer havingtwo transmitters fixed on the pointer at known positions relative to theend. This system calculates the position of the end in response to thepropagation time of a supersonic signal from each of the transmitters toeach of three detectors positioned at known coordinates and positionedso as to have unobstructed propagation paths to each of thetransmitters. In effect, this provides a five-dimensional input device.Further, this disclosure does not teach any particular techniques forefficiently solving the equations involved.

Also, USP 3,792,243 describes a Method for Encoding Positions ofMechanisms, USP 3,792,424 describes Apparatus for Detecting the Positionof a Movable Article Under Water, USP 3,745,519 describes a Small CraftPositioning System and USP 3,383,651 describes a Plane CoordinateComputing System.

SUMMARY OF THE PREFERRED EMBODIMENT OF THE INVENTION

In the preferred embodiment of the present invention signals from anultrasonic transmitter are received by multiple receivers. The receivedsignals are processed by an processing circuit which provides signals toa computer system for use in controlling a computer display or a robot.This system is particularly adapted to use inexpensive components toprovide for a low cost mass producible control device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of one embodiment of the present invention.

FIG. 2 is an illustration of the embodiment of FIG. 1 coupled to acomputer having a display.

FIGS. 2A, 2B and 2C are an illustration of the present invention used asa robot control system.

FIG. 3 is a block diagram of one embodiment of the processing circuitry.

FIG. 4 is a block diagram of the front end circuit of FIG. 3.

FIG. 5 is a block diagram of a second embodiment of the receiver framecircuitry of the present invention.

FIGS. 6A, 6B, 6C, 6D, 6E and 6F are an illustration of the waveforms ofthe signals which characterize FIGS. 3 and 4.

FIG. 7 is a detailed schematic diagram of an ultrasonic receiver, frontend circuit and sample and hold circuit as in FIGS. 3 and 4.

FIG. 8 is a detailed schematic of the circuit used to generate the SNSsignal.

FIG. 9 is a detailed schematic of circuitry used to connect the presentinvention to a computer and to generate the INT signal.

FIG. 10 is a detailed schematic of a circuit to permit the clock circuitto be synchronized to the computer.

FIG. 11 is a detailed schematic diagram of the one shot, oscillator andtransmitter of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE PRESENTINVENTION

FIG. 1 is an illustration of one embodiment of the present invention. Areceiver frame 110 supports three ultrasonic receivers 120, 130 and 140.Receivers 120 and 130 define a first axis, which is orthoganal to theaxis through receivers 130 and 140. The receiver frame 110 is coupled toa hand held ultrasonic transmitter 150 and to a controller port plug160. Circuitry in the receiver frame responds to signals from thereceivers, the transmitter, and the controller port plug and providessignals to the controller port plug indicative of the position oftransmitter 150 relative to the receiver frame.

Receiver frame 110 is constructed in the shape of the letter "L" to fitsecurely onto the top and side of a rectangularly shaped video dispalydevice such as a conventional monitor or television. I have built someframes that are hinged near receiver 130 so that the two arms can foldtogether for more conveinient packaging and storage.

FIG. 2 is an illustration of the embodiment of FIG. 1 coupled to acomputer 200 having a display 210. In this embodiment the receiver frameis adapted to fit on to the display 210 and has been placed on thedisplay and aligned so that the plane of the receiver frame issubstantially parallel to the plane of the video display. Alternativelyit could be integrated into the cabinent of display 210. In thisembodiment the computer is adapted such that the position of thetransmitter is used to control the position of a cursor 220 on thedisplay. More specifically, the receiver frame circuitry determines theposition of the transmitter as projected onto the plane of the display.The computer, in response to position signals from the receiver frameand in accordance with internal instructions from a combination of itshardware, software and firmware responds by providing video displaysignals to display 210 such that moving the transmitter in an "X"direction moves cursor 220 in an "X" direction relative to the displayand moving the transmitter in a "Y" direction moves the cursor in a "Y"direction relative to the display. Controller port 160 has been coupledto the computer I/O connector to receive request and timing signals fromthe computer and to provide the computer with position signals.

As illustrated in FIG. 2, the present invention can be used to move acursor on a display. This movement can be used to draw graphics, toselect options illustrated on the display, or to otherwise set variablesfor a computer program. In general, the present invention can be used toreplace many existing controllers such as mice, graphics tablets andlight pens. For example, the computer 200 could include a video displayscreen which is partitioned into areas corresponding to x and ycoordinates projected into space and having a pointer or cursor image onthe screen having a position on the screen responsive to the projectedposition of the transmitter. By moving the transmitter in space thepointer or cursor can be positioned in certain areas which correspond todifferent options or functions. Thus, by moving the transmitterdifferent options can be selected, and with the addition of a switch tothe transmitter, or by using the third dimension available with thepresent invention, a signal can be transmitted to the computer toactivate the currently selected option. This signal can be transmittedeither electrically along a wire, or transmitted electromagnetically.

In addition, the present invention provides for selective magnificationof the transmitter movements and for generation of signals responsive tomovement of the transmitter in a third dimension.

Alternatively, the present invention could be used to directly control arobot. For example, referring to FIG. 2A, movements of transmitter 150directly control the movements of the hand 225 of a robot arm 220.Movement of the transmitter in an "X" direction relative to the receiverframe 110 causes robot hand 225 to move, by a combination of movementsincluding rotation and extension of its arm, a corresponding amount inthe "x" direction relative to the robots frame of reference. Similarly,movements of the transmitter in the "Y" and "Z" directions causecorresponding movement of robot hand 225 in the "y" and "z" directions.By watching the robot it is relatively simple to make the movements ofthe transmitter needed to effect the desired movements of the robot.Further, the gain, that is, the amplitude of the robot hand movementrelative to the movement of the transmitter, is variable by means ofmagnification control. This permits either fast movements of the robotarm with relatively small transmitter movements of very accuratemovements of the robot arm in response to correspondingly largermovements of the transmitter. In another embodiment the receiver frame110 can be juxtaposed with the frame of reference of the robot.

FIG. 2B illustrates an auxiliary control that can be used to vary themagnification of the present invention. In the illustrated embodimentmagnification control 230 is coupled to the computer. Selecting anincreased magnification causes a correspondingly smaller movement of therobot hand.

FIG. 3 is a block diagram of one embodiment of the processing circuitry.In the preferred embodiment this circuitry is located within receiverframe 110. Oscillator 305 provides an electrical signal 01 having afrequency in the range of 30 to 60 hz. Oscillator 305 is coupled to aone shot 310 which provides pulses having durations of approximately 200usec in response to the leading edges of the 01 signals. Provision isalso also made for external synchronization of oscillator 305 inresponse to a signal on input 312.

The 200 usec pulses from one shot 310 are coupled to enable oscillator315 which produces signals in the ultrasonic range, preferably in therange of 26-200 khz. I have found a 40 khz. oscillator to besatisfactory. The 01 signal is also coupled to reset a ramp generator320 and other portions of the circuitry.

An ultrasonic transmitter 330 is coupled to receiver frame 110 by a wire322 which couples the ultrasonic signal from oscillator 315 to thetransmitter. Alternatively, the transmitter could be battery powered andgenerate the ultrasonic signal internally, synchronizing with the 01signal which could be transmitted by an electromagnetic signal such asan infrared or radio signal from the receiver frame to the transmitter.In yet another alternative the 01 signal could be generated in thetransmitter and a synchronizing signal transmitted to the receiverframe.

Thus, in the present embodiment the 01 signal and the ultrasonic burstsgenerated by the transmitter in response thereto are periodic.Alternatively the transmitter and receiver circuitry responds to anexternally generated signal applied to input 312, which can be aposition request signal from a computer. These position request signalscan be aperiodic or synchronized to the computer or its associateddisplay. Such a position request signal is typically generated by thecomputer once for each display frame (about 30 to 60 times per second).

Preferably the transmitter is a Panasonic piezo device, part numberEFROSB40K2 ultrasonic transducer. This device, and the relatedreceivers, are high-Q devices which have a relatively long rise times incomparison to the period of the ultrasonic signals. Accordingly, a pulsewidth is chosen which is long enough to permit a multiple cycle burst ofultrasonic waves to be generated in response to each 01 signal such thatthe transmitter and receivers reach nominal amplitudes.

Ramp generator 320 produces an electrical signal RAMP characterized by avoltage which increases linearly with the time from 01. The RAMP signalis coupled to an integrator 335 which is also coupled to be reset by the01 signal. Integrator 335 provides an output INT which is the integralof the received RAMP signal from the time of the last 01 signal. Sincethe integral of a ramp function is a square function, the INT signal isalso proportional to the square of the time from the 01 signal.

The ultrasonic signal transmitted from transmitter 330 is received bythree ultrasonic transducers 120, 130 and 140. Preferably thesetransducers are piezo devices of the type sold by Panasonic under thepart number EFRRSB40K2 ultrasonic receiver.

In the preferred embodiment transducers 120, 130 and 140 are positionedsuch that a first axis through the first and second transducersintersects a second axis through the second and third transducers at aright angle. Preferably the three transducers are spaced apartapproximately 12-20 inches. Each transducer is coupled to a front endcircuit which serves to detect the ultrasonic bursts. Specifically,transducer 120 is coupled to a front end circuit 340, transducer 130 iscoupled to a front end circuit 345, and transducer 140 is coupled to afront end circuit 350. These front end circuits are identical and areillustrated in block diagram FIG. 4.

Referring to FIG. 4, the analog output of a transducer is applied to a50 db amplifier 410. I have found that a standard audio amplifier asshown in the 1967 GE Transistor Manual is satisfactory to amplify theultrasonic transducer output. A detector 420 receives the amplifiedtransducer signal and follows the envelope of the 40 khz signal,converting this envelope into a dc voltage. In the preferred embodimentof the present invention detector 420 is simply a rectifier followed bya RC circuit, which passes only the smoothed positive portion of thesignal to produce a dc component which is filtered by the followingcircuitry.

A low pass filter 430 comprises a second order filter having a 3 dbpoint at 4 khz. This filter smooths the envelope from detector 420 andserves to interpolate between successive cycles of the 40 khz waveform.A comparator 440 is coupled to receive the filtered envelope signal andlatches on receipt of the amplitude of the filtered envelope signalexceeding a threshold voltage. In the preferred embodiment thisthreshold is a function of time from the 01 signal and is approximatelyproportional to the inverse square of the time from the 01 signal. Sincethe ultrasonic signal is transmitted in response to the 01 signal andthe ultrasonic signal amplitude in space decreases as the inverse sqaureof distance (and propagation time) this variation in threshold serves toreduce the sensitivity of the receivers to noise. Thus the sensitivityof the receivers increase with time until a signal is received, toaccomodate the weaker signal expected from a more distant transmitter.

Each front end circuit provides a digital REC signal in response todetection of the ultrasonic signal by the comparator. Each of the frontend circuits is coupled to a corresponding sample and hold circuit whichsamples in response to receipt of the REC signal. Front end circuit 340is coupled to sample and hold circuit 355, front end circuit 345 iscoupled to sample and hold circuit 360, and front end circuit 350 iscoupled to sample and hold circuit 365. Each sample and hold circuit isalso coupled to sample the INT signal from integrator 335 in response toreceipt of the REC signal. Thus, sample and hold circuit 355 will sampleand hold the voltage of the INT signal at the instant of receipt of theultrasonic pulse by receiver 120. The amplitude of the sampled INTsignal will be proportional to the square of the time from the 01 signalto the detection of the ultrasonic signal by front end circuit 340.Similarly, sample and hold circuits 360 and 365 will sample and hold theINT signal in response to the detection of the ultrasonic signal byfront end circuits 345 and 350 respectively.

The signals sampled and held by sample and hold circuits 355 and 360 arecoupled to a subtractor circuit 370 which produces a signal V_(x). Theamplitude of V_(x) is proportional to the position of the transmitterrelative to the axis through receivers 120 and 130 as projected onto theplane of the receivers as can be demonstrated by the followingrelationships.

Receivers 120, 130, and 140 are disposed in a plane and the position oftransmitter 150 is defined by its position x, y and z with respect tothe coordinate system having:

Receiver 130 at (-1,-1,0) (One unit negative along the x and y axes)

Receiver 120 at (1,-1,0) (One unit along the x axis, and negative oneunit on the y axis)

Receiver 140 at (-1,1,0) (One unit along the y axis, negative one unitalong the x axis)

In this coordinate system the distance d₁ between the transmitter (atx,y,z) and receiver 130 (at -1,-1,0) becomes:

    d.sub.1 =SQRT[(x+1).sup.2 +(y+1).sup.2 +z.sup.2 ]          (EQ 1)

Similarly, the distance d₂ between the transmitter and receiver 120 (at1,-1,0) becomes:

    d.sub.2 =SQRT[(x-1).sup.2 +(y+1).sup.2 +z.sup.2 ]          (EQ 2)

And, the distance d₃ between the transmitter and reciever 140 (at-1,1,0) becomes:

    d.sub.3 =SQRT[(x+1).sup.2 +(y-1).sup.2 +z.sup.2 ]          (EQ 3)

Solving for x and y yields:

    x=(d.sub.1.sup.2 -d.sub.2.sup.2)/4                         (EQ 4)

    y=(d.sub.1.sup.2 -d.sub.3.sup.2)/4                         (EQ 5)

Equations 4 and 5 give the x and y coordinates of the transmitter interms of the differences of the squares of the distances d₁, d₂, and d₃.Since the distances from the transmitter to the respective receivers areproportional to the propogation times of the ultrasonic pulse to therespective receivers the x and y coordinates of the transmitter becomesa function of the difference in the squares of the propogation times.

Thus, the V_(x) signal provided by subtractor 370 is proportional to thex coordinate of the transmitter. Similarly, the output signal V_(y) fromthe subtractor 375 is proportional to the y coordinate of thetransmitter.

FIG. 5 is a block diagram of a second embodiment of the receiver framecircuitry of the present invention. In this embodiment separateintegrators are provided for each channel. Ramp generator 320 is coupledto an inverting amplifier 510 which provides a negative going rampsignal NRAMP having a slope of the same absolute magnitude as RAMP. TheRAMP signal is switchably coupled through CMOS switch 520 to adder 530in response to the REC1 signal from front end circuit 340. Similarly,the RAMP signal is switchably coupled through CMOS switch 540 to adder550 in response to the REC3 signal from front end circuit 350. Finally,the negative ramp signal NRAMP is coupled through CMOS switch 560 toadders 530 and 550 in response to the REC2 signal from front end circuit345.

Adder 530 adds the applied ramp signals and applies the sum signal tothe input of integrator 570. Thus a positive "x" position of thetransmitter will be characterized by the REC1 signal arriving before theREC2 signal. Similarly, a negative "x" will be characterized by the REC2signal arriving before the REC1 signal. Since the RAMP and NRAMP signalshave equal slopes and start at the same time, once both REC1 and REC2signals have been generated and applied to adder 530 the RAMP and NRAMPsignals will cancel one another and the output of adder 530 will bezero. The integral of the sum signal from adder 530 can be shown to beproportional to the square of the differences between the transmissionstimes which is proportional to the desired "x" position.

A sample and hold circuit 580 samples the integrator output on thereceipt of the second of the two signals REC1 and REC2 and provides asampled analog signal proportional to the x position. The adder 550operates in a similar manner with respect to the "y" position.

FIG. 6 is an illustration of the waveforms of the signals whichcharacterize FIGS. 3 and 4. Specifically, waveform 6a illustrates a 40khz ultrasonic burst. As shown, the ultrasonic burst is characterized bya significant rise and fall time and a multiple number of cycles.Waveform 6b illustrates the output 0 of lowpass filter 430 and thereceived waveform R. Waveform 6c illustrates the output of comparator440. Waveforms 6d, 6e, 6f illustrate the timed relationship between the01 signal 6d, the RAMP signal 6e and the INT signal 6f.

FIG. 7 is a detailed schematic diagram of an ultrasonic receiver, frontend circuit and sample and hold circuit as in FIGS. 3 and 4.Specifically, receiveer 120 receives the ultrasonic burst and is coupledto front end circuit 340. Front end circuit 340 provides a signal F1 inresponse to detecting the ultrasonic burst which causes sample and holdcircuit 355 to sample the signal INT.

More particularly, front end circuit 340 includes an amplifier circuit410 coupled to receive the ultrasonic burst from receiver 120. Amplifiercircuit 410 includes transistors 415 and 417 and provides an amplifiedultrasonic signal to detector 420. Detector 420 comprises a diode 730, acapacitor 740 and a resistor 750, and provides a partially filtered andrectified output to low pass filter 430. Low pass filter 430 includes anoperational amplifier 755 and filters and smooths the signal. Thefiltered output from low pass filter 430 is applied to one input ofcomparator 440 which provides an output signal F1 in response to thefiltered output signal having a voltage greater than the voltage on theSNS input. Comparator 440 includes diode 765 which causes the output ofcomparator circuit 760 to latch in a high state once triggered by theoutput of low pass filter 430. The latch is reset at the end of thecycle when the 01 signal goes low.

Sample and hold circuit 355 samples the INT signal in response to the F1signal and provides a buffered output signal having an amplituderesponsive to the amplitude of the sampled INT signal.

FIG. 8 is a detailed schematic of the circuit used to generate the SNSsignal applied to the second input of comparator 440 of FIG. 7. A 02signal, which is the inverse of the 01 signal, charges capacitor 810,setting a high threshold for detection of a signal by comparator 440.During the 01 signal capacitor 810 discharges through resistors 820 and830 with time such that the voltage of the SNS signal is reduced as afunction of time. Thus, the sensitivity of the comparator 440 increaseswith time from the start of the 01 signal. The RC decay is chosen toapproximate the square-law reduction in signal strength associated withthe distances and propagation times of the transmitter. Thus thecircuitry is less sensitive to noise when the propagation times areshort yet still sensitive enough to detect the ultrasonic burst when thetransmitter is at a considerable distance from the receiver.

FIG. 9 is a detailed schematic of circuitry used to connect the presentinvention to a computer and to generate the INT signal sampled by thecircuitry of FIG. 7. A clock circuit 910 provides 01 and 02 timingsignals for the control of the the initiation of the RAMP signal, thetransmission of the ultrasonic signal, and the discharge and reset ofthe integrator and sample and hold circuits.

In one embodiment of the present invention this oscillator issynchronized to a computer. More specifically, referring to FIG. 10, alink circuit is provided for a computer that permits the clock circuit910 to be synchronized to the computer. Specifically, when the computerreads its "pot" ports it discharges the pot lines in the process. Whenthe XPOT line, which is coupled to the x pot port, is discharged,comparator 1010 discharges forces the 01 line to a low logic state andresets oscillator 910.

Synchronizing the 01 cycle with the reading of the pot lines isadvantageous since by reading the outputs of the processing circuitry ata constant time relative to the transmit times the errors due to driftin the sample and hold circuits are minimized resulting in a steadieroutput signal.

Transistor 970 generates a 02 signal in response to the 01 signal fromclock circuit 910. During the 02 signal capacitor 920 is dischargedthrough transistor 930. During the 01 signal transistor 940 acts as acurrent source and charges capacitor 920, generating the RAMP signal.Similarly, integrator 950 is reset by the 02 signal which connects thenegative input of integrator 950 to ground through switch 960 whichsaturates integrator 950 and resets it to a high output. In operationintegrator 950 will generate the negative integral of the RAMP signal.

FIG. 11 is a detailed schematic diagram of the one shot, oscillator andtransmitter of FIG. 3. One shot 310 enables oscillator 315 whichgenerates a 40 khz signal. Oscillator 315 includes astable multivibrator980 which is coupled to driver transistor 990. The output of drivertransistor 990 is increased in voltage by flyback transformer 992 and isAC coupled by capacitor 995 to ultrasonic transmitter 330.

Transmitter 330 is normally biased to 5 v dc. However, closure of aswitch 997 on the transmitter causes line 322 to go low, which isdetected by circuit 999 which generates a trigger signal TRIG which iscoupled to the computer interface 9010 illustrated in FIG. 9.

In operation line 322 carries a 50 vac 40 khz signal during transmit andalso carries the dc switch signal. In the preferred embodiment coaxialcable is used for this line.

Referring to FIG. 9, an approximation of the z distance is derived fromeither the F1 of F3 signal, whichever occurs first. The F1 and F3signals are coupled to transistor 9020 such that the first occurring ofthese signals generates a ZBAR signal, applied to the computer interface9010. The time of this signal relative to 01 provides an approximatemeasure of the z position of the transmitter. Alternatively, z can beobtained exactly either by use of a more complex algorithm or circuitryor by using a single additional sensor in the z dimension. However, Ihave found that these alternatives are slow, complex and involve moreexpense than the preferred embodiment.

Referring to FIG. 10, adder circuit 1100 provides a difference signal inresponse to the INT signals sampled by front end circuits 340 and 345.Similarly, circuit 1200 provides a difference signal in response to theINT signals sampled by front end circuits 345 and 350. These differencesignals are then buffered and processed for application to the interfaceconnection 9010 of FIG. 9 and presented to the computer as x and ypotentiometer inputs.

As can be appreciated from the above descriptions the present inventionis sensitive only to the first signals from the transmitter since itimmediately latches in response to the detection of the ultrasonicburst. I have determined that echos have substantially died in 16 to 33msec, thus I have used that time as my preferred repetition rate. Thecombination of this repetition rate and the variable threshold circuitrycombines to make the present circuitry quite insensitive to noise.

While the invention has been particularly taught and described withreference to the preferred embodiments, those versed in the art willappreciate that minor modifications in form and details may be madewithout departing from the spirit and scope of the invention. Forinstance, in the embodiment illustrated in FIG. 5 it would be equivelentto generate only a positive ramp signal and to replace the adders with asubtractor circuit. Accordingly, all such modifications are embodiedwithin the scope of this patent as properly come within my contributionto the art and are particularly pointed out by the following claims.

I claim:
 1. A system for indicating the position of a hand movableultrasonic transmitter with respect to an array of detectorscomprising:an ultrasonic transducer for transmitting a burst ofultrasonic waves; an array of detectors responsive to the burstincluding three point detectors positioned such that a first axisthrough the first and second detectors intersects a second axis throughthe second and third detectors; means responsive to the detection of theburst by the first and second detectors for indicating the position ofthe transmitter relative to the first axis and responsive to thedetection of the burst by the second and third detectors for indicatingthe position of the transmitter relative to the second axis andresponsive to the detection of the burst by only one of the detectorsfor indicating the position of the transmitter relative to a third axisorthogonal to the plane defined by the three detectors.
 2. A system asin claim 1 wherein the one detector is the detector which first receivesthe burst.
 3. Apparatus for controlling the position of a cursor on adisplay screen comprising:means for generating a first signal; a handmovable ultrasonic transducer for transmitting a burst of ultrasonicwaves in timed relationship to the first signal; an array of detectorsresponsive to the burst including three point detectors positioned suchthat a first axis through the first and second detectors intersects asecond axis through the second and third detectors at substantially aright angle; means for providing a second signal proportional to thesquare of the period of time from the generation of first signal; meansfor sampling the second signal in response to the detection of the burstby each of the detectors; first means responsive to the signal sampledin response to the detection of the burst by the first and seconddetectors for indicating the position of the transmitter relative to thefirst axis; second means responsive to the signal sampled in response tothe detection of the burst by the second and third detectors forindicating the position of the transmitter relative to the second axis;and display means for positioning a cursor on a display screen relativeto one axis of the display screen in response to the position of thetransmitter relative to the first axis and for positioning the cursor onthe display screen relative to another axis of the display screen inresponse to the position of the transmitter relative to the second axis.4. Apparatus as in claim 3 wherein the second signal is an analogvoltage.
 5. Apparatus as in claim 3 wherein the second signal is abinary value in a digital counter.
 6. Apparatus as in claim 3 whereinthe second signal is represented by a binary value stored in a memorylocation of a general purpose microprocessor.
 7. Apparatus as in claim 3wherein the hand movable ultrasonic transducer also transmits the firstsignal electromagnetically to the means for providing the second signal.8. Apparatus as in claim 3 further comprising means for generating thefirst signal and for transmitting an electromagnetic signal to theultrasonic transducer in timed relationship to the first signal, whereinthe ultrasonic transducer transmits the burst in response to theelectromagnetic signal.
 9. Apparatus for controlling the position of acursor on a display screen comprising:means for generating a firstsignal; a hand removable ultrasonic transducer for transmitting a burstof ultrasonic waves in timed relationship to the first signal; an arrayof detectors responsive to the burst including three point detectorspositioned such that a first axis through the first and second detectorsintersects a second axis through the second and third detectors atsubstantially a right angle; means for providing a ramp signalproportional to the period of time from the generation of the firstsignal; means for integrating the ramp signal from a first detection ofthe burst by one detector until a subsequent detection by anotherdetector on the same axis and for providing an integral in response tothe detections by the first and second detectors and another integral inresponse to the detections by the second and third detectors; firstmeans responsive to the integral provided in response to the detectionsby the first and second detectors for indicating the position of thetransducer relative to the first axis; second means responsive to theintegral provided in response to the detections by the second and thirddetectors for indicating the position of the transducer relative to thesecond axis; and display means for positioning a cursor on a displayscreen relative to one axis of the display screen in response to theposition of the transmitter relative to the first axis and forpositioning the cursor on the display screen relative to another axis ofthe display screen in response to the position of the transmitterrelative to the second axis.
 10. Apparatus as in claim 9 wherein theramp signal is latched to an integrator in response to the detection ofthe burst by one detector and further comprising:means for inverting theramp signal; and means for latching the inverted signal to the input ofthe integrator in response to detection of the burst by the otherdetector on the same axis as the one detector.
 11. Apparatus as in claim9 wherein the hand movable ultrasonic transducer also transmits thefirst signal electromagnetically to the means for providing the rampsignal.
 12. Apparatus as in claim 9 further comprising means fortransmitting an electromagnetic signal to the ultrasonic transducer intimed relationship to the first signal, wherein the ultrasonictransducer transmits the burst in response to the electromagneticsignal.
 13. A method of determining the position of an ultrasonictransmitter with respect to an axis having two point receiverspositioned thereon comprising the following steps:transmitting a burstof ultrasonic waves from the transmitter; generating a representationproportional to the square of the period of time from the transmission;detecting the receipt of the burst at each of the receivers; samplingthe representation at the time of each detection; and generating a valueindicative of the position in response to the difference between thesampled representations.
 14. A method as in claim 13 for furtherdetermining the position of the ultrasonic transmitter with respect toanother axis through one of the two receivers and a third receivercomprising the additional steps of:detecting the receipt of the burst atthe third receiver; sampling the representation at the time of detectionby the third receiver; and generating a value indicative of the positionwith respect to the other axis in response to the difference between therepresentations sampled in response to the detections by the the one andthe third receivers.
 15. A method as in claim 14 wherein the step ofgenerating a representation proportional to the square of the period oftime from transmission comprises the steps of:generating an intermediaterepresentation proportional to the period of time from transmission; andintegrating the intermediate representation.
 16. A method as in claim 15wherein the intermediate representation is an analog signal and the stepof integrating is performed by an analog integration circuit.
 17. Amethod as in claim 15 wherein the intermediate representation is abinary number stored in a register and the step of integrating isperformed by an accumulator circuit.
 18. A method as in claim 15 whereinthe intermediate representation is a time value in a general purposemicroprocessor and the step of integrating is performed by periodicallyadding the time value to a memory location.
 19. A method of determiningthe position of an ultrasonic transmitter with respect to an axis havingfirst and second point receivers positioned thereon comprising thefollowing steps:transmitting a number of cycles of an ultrasonic wavefrom a transmitter; detecting the receipt of the burst at each of thereceivers; generating a positive representation proportional to theperiod of time from transmission to detection by the first receiver;generating a negative representation proportional to the period of timefrom transmission to the detection by the second receiver; integratingthe sum of the representations; and sampling the integrated sum afterdetection of the burst by both detectors.
 20. A method as in claim 19wherein the positive and negative representations are analog signals andthe step of integrating is performed by an analog integration circuit.21. A method as in claim 19 wherein the positive and negativerepresentations are binary numbers stored in a register and the step ofintegrating is performed by an accumulator circuit.
 22. A method as inclaim 19 wherein the intermediate representation is a time value in ageneral purpose microprocessor and the step of integrating is performedby periodically adding the time value to a memory location. 23.Apparatus as in claim 3 further comprising;a switch located on the handmovable ultrasonic transducer for generating a switch signal; wirescoupled between the transducer and the first and second means forproviding ac drive to the transducer; means for transmitting the switchsignal as a dc level shift on the wires; and means for detecting theswitch signal on the wires for providing a selection signal. 24.Apparatus as in claim 9 further comprising;a switch located on the handmovable ultrasonic transducer for generating a switch signal; wirescoupled between the transducer and the first and second means forproviding ac drive to the transducer; means for transmitting the switchsignal as a dc level shift on the wires; and means for detecting theswitch signal on the wires for providing a selection signal.
 25. Asystem for programming movements of a robot in a first coordinate systemin response to movements of a hand movable ultrasonic transmitter in asecond coordinate system defined by an array of detectors comprising:anultrasonic transducer for transmitting a burst of ultrasonic waves; anarray of detectors responsive to the burst including three pointdetectors positioned such that a first axis through the first and seconddetectors intersects a second axis through the second and thirddetectors; means responsive to the detection of the burst by the firstand second detectors for indicating the position of the transmitterrelative to the first axis and responsive to the detection of the burstby the second and third detectors for indicating the position of thetransmitter relative to the second axis and responsive to one of thedetectors for indicating the distance between the transmitter and one ormore detectors which provides the position of the transmitter relativeto a third axis orthogonal to the plane defined by the three detectors;means for controlling the movements of a robot such that movements ofthe transmitter along the first, second and third axes of the secondcoordinate system result in movements of the robot in corresponding axesof the first coordinate system.
 26. Apparatus as in claim 25 furthercomprising means coupled to the ultrasonic transducer for synchronizingthe transmitting of the burst of ultrasonic waves with a request signalfrom a computer.